[en] Porous solids built of fractal clusters can be obtained by destabilization of quasi-monodisperse silica sols. Small-angle neutron scattering, associated with transmission-electron microscopy, was used for a structural investigation of these aerogels in a broad length scale. We present a quantitative analysis of the data for materials with different particle sizes and polydispersities. The analysis of the wavector region corresponding to the crossover between surface and volume scattering is especially emphasized. (orig.)

[en] Brillouin scattering experiments have been performed from 5 to 1600 K in vycor, a porous silica glass. The acoustic velocity and attenuation at hypersonic frequencies are compared to those of bulk silica and others porous silica samples. The experimental evidence for the influence of porosity on the scattering by acoustic waves is compared to calculations. The correlation between internal friction and thermal conductivity at low temperature is discussed

[en] Complete text of publication follows. Coherent inelastic neutron (INS) and light scattering measurements on silica xelogels over a wide range of densities and temperatures are presented Raman scattering (RS) results show the presence of a Boson Peak (BP) at temperatures where the quasielastic (QES) contribution is negligible. The BP of xerogels is at a frequency around 35 cm^-1 and has a shape similar to that of melted silica. Another bump at 7 cm^-1 is also visible in RS, which is connected to the presence of the pores. While BP is also revealed by INS, this second peak is not. This important result seems to indicate that the disorder introduced by the presence of the pores doesn't influence the density of states, and therefore is not seen by neutrons, but only by RS (apparently sensitive to details of the morphology). A comparison between the Q-dependence of the elastic and inelastic structure factors as a function of sample density and temperature is also presented. The absence of a peak in the inelastic structure factor for the Q value corresponding to the first sharp diffraction peak demonstrates the existence of random-phase modes for energies around the BP. (author)

[en] Complete text of publication follows. An experimental investigation of the relaxational processes related to the glass transition in several glass formers with more or less complex molecular architecture is presented. This inelastic neutron scattering study concentrates on the region around 1.1 to 1.5 T_g where the two relaxation processes usually identified in most glass formers, the α and the β relaxations, are expected to merge or cross. A recent study comparing the dynamics of Sorbitol and Maltitol (two low molecular and complementary glasses) seems to show that the way on which the α and β processes merge depends on the differences in the chemical architecture of these polyols [1]. In the present work, linear diols, three-arm-star triols and crosslinked polyurethanes, synthesized from the latter are studied. This work is an extension of a previous study of the relaxational processes in cross-linked polyurethanes [2]. For such series of samples of similar chemical composition but with increasing complexity in the architecture, the influence of the molecular complexity on the type of merging between α and β processes is tested. This allows to discuss the α-β cross-over on molecular level. (author)

[en] In order to determine the influence of the thermal history (fictive temperature) and OH content on the elastic properties of silica glass, we have investigated high resolution in situ Brillouin experiments on SiO₂ glass from room temperature to the supercooled liquid at 1773 K across the glass transition. The well known anomalous increase of elastic modulus in the glassy state and in the supercooled liquid regime is observed. No change in the slope of the elastic moduli of silica appears as a characteristic of the glass transition, in contrast to what happens in various other glasses. We show that thermal history has a weak effect on elastic moduli in the glass transition regime for silica glass. The effect of the water content in silica glass is greater than the fictive temperature effect and gives larger changes in the amplitude of the elastic modulus for the same thermal dependence. A singular decrease above 1223 K is also observed in the shear moduli for hydrated samples. Different models explaining the temperature dependence of the elastic properties in relationship with frozen-in density fluctuations or with the structure are discussed